1. Field of the Invention
The present invention relates to a vacuum column web loop position sensing system, and more particularly, to an optical sensing system used to detect the position of magnetic tape in a vacuum column in which light is emitted from behind the vacuum column and reflected from the bottom of the vacuum column back towards light receptors also positioned behind the vacuum column.
2. Description of the Related Art
Vacuum columns are commonly used to maintain a constant amount of tension on web material, such as magnetic tape, when the web material is transported from one reel to another. The purpose of using such columns is to provide slack in the web material to allow for differences in speed of rotation of the supply and take-up reels to prevent breaking without permitting the web material to become snarled. In order for the vacuum column to perform these functions, it is necessary that the web material form a loop between the bottom and the mouth of the vacuum column.
Prior art devices are known for detecting the position of a tape loop in magnetic tape devices, such as recorders, players, readers, etc., to ensure that the vacuum column is able to perform its functions. Examples of prior art tape loop sensors include a series of discrete sensing elements, such as the ports in the vacuum sensing devices disclosed in U.S. Pat. Nos. 4,509,007 to Barsotti et al. and 3,866,855 to Bryer, the strips of solar cells positioned along the wear surface of a vacuum column disclosed in U.S. Pat. No. 3,898,693 to Chang and a multi-prism bar disclosed in U.S. Pat. No. 3,791,607 to Klang et al. Optical sensors are also used in the prior art to detect a transparent leader, as disclosed in U.S. Pat. No. 4,616,274 to Nagaoka, and the position of a mechanical tensioning device used instead of a vacuum column, as disclosed in U.S. Pat. No. 4,104,685 to Chang.
However, these prior art devices have several drawbacks. When an array of discrete sensors is used, a step voltage is produced resulting in a control signal which is not smooth. The prior art devices typically rely upon the positioning of light receptors (photocells) along the side of a vacuum column upon which the tape slides. Whether these light receptors are photocells or a multi-prism bar, they should be protected from the tape that slides over the light receptors while retaining transparency to permit light to pass therethrough. In one prior art device, this is accomplished by drilling holes in metal spacers and positioning the light receptors (and light emitters) in the hole so that they do not contact the tape. This is an expensive manufacturing process and requires careful alignment of the light emitters and receptors.
In addition, it is necessary to protect the light receptors from stray light. Typically, stray light protection requires an opaque cover or shield for the vacuum column which prevents visual confirmation of tape loop position and adds to the cost. Also, the use of discrete light receptors requires that the cost of providing multiple receptors must be offset by the resolution of the tape's position which is desired. For example, a sensing system using five light receptors is inexpensive, but only provides an indication of five possible positions of the tape loop, while using 50 sensors provides a more precise indication of the tape loop position, but is ten times as expensive.